Introduce Variable: Support extraction of string template fragments

#KT-2089 Fixed

idea/testData/refactoring/introduceVariable/stringTemplates/fullEntryWithSimpleName.kt.after

@@ -0,0 +1,7 @@
+fun foo(a: Int): String {
+    val a1 = a
+    val x = "-$a1"
+    val y = "x${a1}y"
+    val z = "x$ay"
+    return "abc${a1}def"
+}